feat(paper-trading): forward-test ledger engine

Adds a background forward-test engine that records flat-stake "paper" bets
for directional anomalies as they fire and settles them when results arrive,
measuring the detector's live, out-of-sample edge — the antidote to backtest
overfitting. The results UI is a follow-up.

- Domain: PaperBet entity (Rate>1 / Stake>0 invariants, Open factory,
  SettleAgainst — Won pays stake x rate, else Lost) + AnomalyEvidenceSide.RateFor.
- Application: OpenPaperBetsUseCase (directional + score gate, dedups by
  AnomalyId, picks the post-flip favourite and its locked-in rate) and
  SettlePaperBetsUseCase (Won when pick == winner else Lost; ungraded events
  stay open; batched result lookup).
- Infrastructure: PaperBetEntity + config (TEXT decimals, unique AnomalyId index,
  Outcome index), repository, mapping, additive AddPaperBets migration, and
  PaperTradingWorker (config-gated, baseline since-marker, open+settle per cycle).
- Config: PaperTradingOptions / appsettings PaperTrading (Enabled:false default).
- 25 tests: domain settlement, both use cases, and a real-SQLite round-trip
  incl. the unique-AnomalyId double-open backstop.

src/Marathon.Application/UseCases/OpenPaperBetsUseCase.cs

@@ -0,0 +1,84 @@
+using Marathon.Application.Abstractions;
+using Marathon.Domain.AnomalyDetection;
+using Marathon.Domain.Entities;
+using Marathon.Domain.Enums;
+using Microsoft.Extensions.Logging;
+
+namespace Marathon.Application.UseCases;
+
+/// <summary>
+/// Opens flat-stake paper bets for directional anomalies detected in
+/// (<c>since</c>..<c>until</c>] whose score clears the threshold and that don't
+/// already have one. The picked side is the post-flip favourite; the rate is that
+/// side's post-suspension rate — locking in the price the moment the signal fired.
+/// </summary>
+public sealed class OpenPaperBetsUseCase
+{
+    private readonly IAnomalyRepository _anomalies;
+    private readonly IPaperBetRepository _paperBets;
+    private readonly ILogger<OpenPaperBetsUseCase> _logger;
+
+    public OpenPaperBetsUseCase(
+        IAnomalyRepository anomalies,
+        IPaperBetRepository paperBets,
+        ILogger<OpenPaperBetsUseCase> logger)
+    {
+        _anomalies = anomalies ?? throw new ArgumentNullException(nameof(anomalies));
+        _paperBets = paperBets ?? throw new ArgumentNullException(nameof(paperBets));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    /// <summary>Returns the paper bets opened this pass (empty when nothing qualified).</summary>
+    public async Task<IReadOnlyList<PaperBet>> ExecuteAsync(
+        DateTimeOffset since,
+        DateTimeOffset until,
+        decimal minScore,
+        decimal flatStake,
+        CancellationToken ct = default)
+    {
+        if (flatStake <= 0m)
+            throw new ArgumentOutOfRangeException(nameof(flatStake), flatStake, "Flat stake must be positive.");
+
+        var anomalies = await _anomalies.ListByDateRangeAsync(since, until, ct).ConfigureAwait(false);
+
+        // Only directional kinds make a side prediction worth forward-testing; the rest
+        // are informational and would just measure the base favourite-win rate.
+        var candidates = anomalies
+            .Where(a => a.Kind.IsDirectional() && a.Score >= minScore)
+            .ToList();
+        if (candidates.Count == 0)
+            return Array.Empty<PaperBet>();
+
+        var existing = await _paperBets
+            .GetExistingAnomalyIdsAsync(candidates.Select(a => a.Id).ToList(), ct)
+            .ConfigureAwait(false);
+
+        var opened = new List<PaperBet>();
+        foreach (var anomaly in candidates)
+        {
+            ct.ThrowIfCancellationRequested();
+
+            if (existing.Contains(anomaly.Id))
+                continue;
+
+            if (!AnomalyEvidenceParser.TryParse(anomaly.EvidenceJson, out var evidence))
+                continue;
+
+            var pick = evidence.PostSuspension.Favourite;
+            if (evidence.PostSuspension.RateFor(pick) is not { } rate || rate <= 1m)
+                continue;
+
+            opened.Add(PaperBet.Open(anomaly.Id, anomaly.EventId, pick, rate, flatStake, anomaly.DetectedAt));
+        }
+
+        if (opened.Count == 0)
+            return Array.Empty<PaperBet>();
+
+        foreach (var bet in opened)
+            await _paperBets.AddAsync(bet, ct).ConfigureAwait(false);
+        await _paperBets.SaveChangesAsync(ct).ConfigureAwait(false);
+
+        _logger.LogInformation("OpenPaperBetsUseCase: opened {Count} paper bet(s)", opened.Count);
+        return opened;
+    }
+}

src/Marathon.Application/UseCases/SettlePaperBetsUseCase.cs

@@ -0,0 +1,61 @@
+using Marathon.Application.Abstractions;
+using Marathon.Domain.Enums;
+using Marathon.Domain.ValueObjects;
+using Microsoft.Extensions.Logging;
+
+namespace Marathon.Application.UseCases;
+
+/// <summary>
+/// Settles every open (<see cref="BetOutcome.Pending"/>) paper bet whose event now has
+/// a final result — Won when the picked side matches the winner, otherwise Lost. Bets
+/// on events that aren't graded yet stay open and are retried next cycle.
+/// </summary>
+public sealed class SettlePaperBetsUseCase
+{
+    private readonly IPaperBetRepository _paperBets;
+    private readonly IResultRepository _results;
+    private readonly ILogger<SettlePaperBetsUseCase> _logger;
+
+    public SettlePaperBetsUseCase(
+        IPaperBetRepository paperBets,
+        IResultRepository results,
+        ILogger<SettlePaperBetsUseCase> logger)
+    {
+        _paperBets = paperBets ?? throw new ArgumentNullException(nameof(paperBets));
+        _results = results ?? throw new ArgumentNullException(nameof(results));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    /// <summary>Returns the number of paper bets settled this pass.</summary>
+    public async Task<int> ExecuteAsync(CancellationToken ct = default)
+    {
+        var open = await _paperBets.ListByOutcomeAsync(BetOutcome.Pending, ct).ConfigureAwait(false);
+        if (open.Count == 0)
+            return 0;
+
+        // Batched result lookup — one query, not one per open bet.
+        var eventIds = open.Select(b => b.EventId).Distinct().ToList();
+        var results = await _results.GetManyAsync(eventIds, ct).ConfigureAwait(false);
+
+        var settledAt = MoscowTime.Now;
+        var settled = 0;
+        foreach (var bet in open)
+        {
+            ct.ThrowIfCancellationRequested();
+
+            if (!results.TryGetValue(bet.EventId, out var result))
+                continue; // event not graded yet
+
+            await _paperBets.UpdateAsync(bet.SettleAgainst(result.WinnerSide, settledAt), ct).ConfigureAwait(false);
+            settled++;
+        }
+
+        if (settled > 0)
+        {
+            await _paperBets.SaveChangesAsync(ct).ConfigureAwait(false);
+            _logger.LogInformation("SettlePaperBetsUseCase: settled {Count} paper bet(s)", settled);
+        }
+
+        return settled;
+    }
+}